1. Field of the Invention
The present invention relates to fume dust suppression and to a fume dust prevention method for use during handling of molten metal. An important object is to prevent generation of fume dust from a vessel when molten metal containing carbon is poured into a vessel such as a ladle for containing molten iron or steel, for example.
2. Description of the Related Art
It is known that dangerous quantities of fume dust generate during pouring of molten metals such as molten iron or steel, for example. This happens during transfer of molten metal from a vessel (or equipment) to any other vessel (or other equipment). Dust, scattered by the fume dust, has a negative effect on the working environment and on the peripheral environment. A dust collector has been conventionally used as a protective measure, but its effect is unfortunately limited.
Methods of decreasing the generation of fume dust include, for instance, those described in Japanese Unexamined Patent Application Publication No. 49-9405 (mentioned as a first xe2x80x9cconventional examplexe2x80x9d hereinafter) and Japanese Unexamined Patent Application Publication No. 9-96492 (mentioned as a xe2x80x9csecond conventional examplexe2x80x9d hereinafter).
The first conventional example discloses the idea of introducing an inactive gas or spray water into a ladle, and then discharging the molten iron into the ladle.
The second conventional example discloses pouring molten iron into a container after an inactive gas has been fed into the container prior to the pouring. This is intended to inactivate the atmosphere in the container by removal of air from the container. Then molten iron is poured while the inactive gas is continuously supplied into the container.
However, in the aforementioned first and second conventional examples, the large amount of about 20,000 Nm3/H of inactive gas is required to prevent fume dust when the molten metal is poured into, for instance, a ladle of about 150 ton capacity. Thus, these procedures are highly uneconomical and impractical. Moreover, the excessively large amount of inactive gas has to be provided instantaneously. It is difficult to supply such a large amount of inactive gas with stability, and is basically impossible to execute commercially.
The aforementioned first conventional example discloses a method in which water spray is used, instead of directly supplying an inactive gas. Accordingly, steam generated by the heat of molten metal or the like is desired to be used as an inactive gas to prevent the generation of fume dust. However, dangerous steam explosion is threatened due to contact between water and the hot molten object during operation, causing serious safety problems.
Moreover, the required amounts of water spray to form an effective inactive gas atmosphere are not well understood; accordingly, the use of excessive water spray is necessary. Thus, the method faces unresolved problems such as increased operation costs and equipment capacity, and increased danger of steam explosions.
Accordingly, it is an object of the present invention to provide a fume dust suppression or prevention, so that handling of molten metal can be free of fume dust, by controlling water spray amounts, or water spray methods.
In this invention, we preferably start or finish water (or water mist) spraying in accordance with the timing (starting or finishing) of the molten metal pouring. Water dripping from the water spray device to the vessel is prevented by supplying gas (normally purge gas) into the water spray device when the water pressure at the beginning and ending of water supply into the water spray device is unstable. Steam explosion may be thus avoided. It is preferable that the supply of gas is essentially stopped and water is sprayed during the pouring to efficiently prevent fume dust.
Theretore, the most preterable sequence in accordance with this invention is to introduce water mist by supplying water and gas into a water spray device; start pouring the molten metal; then switch on the water spray; restart the gas supply before completion of the molten metal pouring step; and thereby convert to water mist; and subsequently finish pouring the molten metal.
Furthermore, at the beginning of supplying water spray or water mist, gas is preferably supplied prior to the water supply, thereby preventing water drops from dripping as well as preventing steam explosion. In this case, the water supply is preferably started when the water feed pressure is sufficiently high, thus more effectively preventing water dripping.
It is also preferable, after the spraying, to purge residual water from the water feed system (in other words, the water spray device and piping line connected thereto) by supplying gas even after the end of the step of water supply. Due to this operation, water dripping is prevented and steam explosion is avoided.
Moreover, the present invention prevents steam explosion by selecting the particle size of the spraying water particles in the vaporized state at the time of spraying it into the molten metal. As a specific method, the particle size of spray water particles can be calculated on the basis of the distance between the spraying location and the molten metal surface, as will be explained in further detail hereinafter.
Furthermore, it is preferable in the practice of the present invention to spray water or water mist into the molten metal flow pouring into the receiving vessel. In this case, steam flows along the molten metal while the molten metal is flowing, thus effectively reducing oxygen at the surface of the molten metal or inside the vessel.